This invention relates to a microwave oscillator circuit with a field effect transistor (hereinafter simply referred to as FET) and more particularly, to an improvement of a microwave oscillator circuit which is constituted by a micro-strip line, a dielectric resonator and an FET and which is suitable for use in, for example, a converter for conversion of SHF band TV signals into UHF band signals.
For example, U.S. patent application Ser. No. 84458 (corresponding to West German patent application No. P 2941826.6), now U.S. Pat. No. 4,307,352, issued Dec. 22, 1981 and titled "Micro-Strip Oscillator with Dielectric Resonator discloses a basic arrangement of a microwave oscillator circuit with a micro-strip line, a dielectric resonator and an FET. The microwave oscillator circuit with the micro-strip line and dielectric resonator, because of its relatively simplified circuit arrangement, has recently been applied to an SHF converter.
Illustrated in FIG. 1 is a prior art micro-wave oscillator circuit with an FET, a micro-strip line and a dielectric resonator. This oscillator circuit comprises a micro-strip line consisting of a stub 3, an output line 5, a filter 7 connected to the output line 5 and a gate line 10, an FET 1, a dielectric resonator 11, a bias resistor 6, and an end resistor 9, the FET 1 having its source terminal 4 connected to the output line 5, its drain terminal 2 connected to the stub 3 and its gate terminal 8 connected to the gate line 10. The bias resistor 6 is connected to the filter 7 and the end resistor 9 is connected to the gate line 10, thus setting up a current path for DC bias current to flow through the FET 1. The dielectric resonator 11 is disposed adjacent to the gate line 10 and electrically coupled therewith to constitute a resonant circuit. The stub 3 serves as a short-circuiting stub for 1/4 wave-length so that the drain terminal 2 of the FET 1 is grounded in terms of high frequencies. In this oscillator circuit, an internal capacitance between the source and gate electrodes of the FET 1 acts as a feedback capacitance.
In the oscillator circuit of the above construction, when a power supply voltage V.sub.D is applied to a voltage supply terminal 12, this circuit oscillates with the voltage V.sub.D ranging from 2 to 10 volts to deliver an oscillating output by way of the output line 5. However, the oscillation frequency of this oscillator circuit fluctuates to a great extent when the power supply voltage V.sub.D fluctuates. For example, under the condition that the circuit is applied with a power supply voltage V.sub.D to 7 volts to oscillate at a frequency of 11 GHz, the oscillation frequency fluctuates by about 50 KHz/0.1 volt. Further, the oscillation frequency also fluctuates to a great extent when the ambient temperature varies. For example, the oscillation frequency is effected by temperature variations of the FET 1 to fluctuate by about 10 KHz/.degree.C. However, it is necessary for a microwave oscillator, as applied to an SHF converter for reception of TV signals of about 12 GHz, to have an oscillation frequency fluctuation which is less than 300 KHz. Usually, the SHF converter is required to operate under temperature variations of from -10.degree. C. to +50.degree. C. When it comes to satisfying these requirements, the conventional oscillator circuit is not always satisfactory and is unsuitable for incorporation into the SHF converter.
In order to suppress the oscillation frequency fluctuation to below 5 KHz/.degree.C., it is conceivable that a dielectric resonator be chosen which has temperature characteristics that cancel out the oscillation frequency fluctuation due to temperature variations of the FET. However, to cancel out different oscillation frequency fluctuations attributable to different FETs, it is necessary to choose a dielectric resonator having specified temperature characteristics meeting a specified FET used. Further, since the oscillation frequency fluctuation due to the power supply voltage fluctuation is affected by individuality of the FETs, it is difficult for the prior art microwave oscillator circuit to suppress the oscillation frequency fluctuation.